(1) Field of the Invention
The invention relates to communications networks in general and more particularly to protocol for stripping data from the network.
(2) Prior Art
The use of serial or ring communication networks for transferring data is well known in the prior art. Such networks consist of a set of stations serially connected to a communication medium. Information is transferred sequentially, bit by bit, from one station to the next. The inactive stations are bypassed while the active stations regenerate and repeat each bit. Furthermore, the stations are used as the means for attaching one or more devices (terminals, work station, computers, display, etc.) which communicate with other devices on the network.
An originating station transfers its information (message) onto the ring. The message circulates, passes one or more intermediate stations, until it arrives at the destination station where it is copied. In some implementations, the destination station strips the message from the ring. In other implementations, the message remains on the ring until it arrives at the originating station where it is stripped from the ring. Usually, the ring also includes a monitor station which is responsible for stripping messages that have not been stripped by the station normally responsible to perform the stripping. In addition, the monitor station may perform other types of housekeeping functions for the ring.
An access method protocol is a necessary element for the above-described ring network. The protocols are the mechanisms which enable a station to transmit on the ring. Stated another way, an access protocol gives a station the right to place its messages for transmission on the ring.
The prior art uses several different types of access method protocols. The so-called "register" insert access protocol and "token" insert access protocols are representative of the prior art. Whenever the register insert access protocol is used a station seizes the opportunity to transmit when there is an "idle" signal on the ring. With the token insert access protocol a station has an opportunity to transmit whenever it receives a free token. The free token is usually generated by an upstream station.
Although the prior art access method protocols work well for the intended purpose, they are plagued by certain defects which tend to reduce the overall efficiency of the ring networks. The register insert protocols require the introduction of relatively long delays in the ring in order that expendable messages can be successfully stripped from the ring. The delay lengthens the time that is needed to process a message. Likewise, the "token" access protocol allows only one message to be on the ring at any particular instant of time. When the messages are relatively short, compared to the latency of the ring, only a fraction of the available bandwidth is used. Failure to use all of the available bandwidth reduces the efficiency of the ring because a series of idle patterns must be transmitted to fill up the vacancy on the ring.
Another requirement for the above described communication networks is that message fragments and exhausted or expendable messages must be removed or stripped from the ring. The message stripping can take place at some time after the message has reached its destination node. Stripping generally involves either replacing the stripped message with a new message or else replacing the old message with idle patterns. On a register-insertion ring it is desirable to begin stripping early, since this tends to recover a portion of the ring's bandwidth for reuse by another transmitting station.
In a ring which has a latency that greatly exceeds the duration of the shortest message, and which ring employs an access protocol that allows multiple messages to coexist on the ring, it is desirable for each station that has originated a message to strip this message, after the message has circulated once around the ring, such that the message will not be accepted a second time by a destination station.
A station must be able to distinguish between messages that require stripping actions by this station and those messages which require stripping actions by another station. In order to make this distinction, some serial buffering of a message may be required. It is desirable to keep such buffering to a minimum in order to reduce costs and to reduce added ring latency.
The prior art uses different types of stripping protocols to dispose of exhausted messages and/or fragments. One stripping protocol requires the target station to strip each message that it receives. This approach recovers bandwidth promptly. However, this approach is unsuitable for broadcast messages, destined for multiple target nodes. Also, this approach does not provide feedback to the original sending station.
Another stripping protocol requires that the sending station strip each message which it has sent, after this message comes back around the ring. This approach is suitable when the access protocol is such that no messages originated by another station can arrive, at the sending station, prior to the return of any messages which this sending station has originated. This approach can also be used if the sending station is designed so as to insert sufficient buffer delay into the ring such that the sending station can examine the source address of an arriving message, and then determine whether this arriving message should be stripped, or else should be permitted to continue on its way around the ring. Since the location of the source address field may occcur many bytes after the start delimiter field, this approach adds undesirable delay to the ring and also requires the addition of costly buffering facilities for each station.
It is worthwhile noting that the prior art uses a single station to strip expendable messages from the ring. Due to the single station approach the above-described prior art stripping techniques are plagued with the above-described problems.